1. Field of the Invention
The present invention relates to a polarization-direction-controlling element and an exposure device. More specifically, it relates to a polarization-direction-controlling element and an exposure device used in an exposing-recording device for forming an image by scanning a recording medium with light outputted from a light source.
2. Description of the Related Art
An exposing-recording device for recording a two-dimensional image on a photosensitive material by rotating a drum with the photosensitive material (recording medium) mounted on the outer circumferential surface in a main scanning direction and directing a laser beam modulated according to image data of an image to be recorded on the photosensitive material for scanning in a sub scanning direction orthogonal to the main scanning direction, has been conventionally used.
In this kind of the exposing-recording device, in order to record an image with a low resolution, a method in which a spot size of the laser beam on the photosensitive material surface is reduced and a recording pitch in the sub scanning direction is widened, or a pixel of the same image data is repeatedly recorded for reduction of the resolution without change in the spot size or the recording pitch. In contrast, in order to record an image with a high resolution, an opposite method is used.
In order to enlarge or reduce the laser beam spot size, however, a lens of an optical system, or the like should be driven by a driving mechanism, and thus there is a problem in that the device becomes bulky and the costs are increased. Moreover, when a pixel of the same image data is repeatedly recorded in order to reduce the resolution, the recording pitch in the sub scanning direction is constant, and thus there is a problem in that a recording speed cannot be improved.
Therefore, in order to solve these problems, according to the technique disclosed in the official gazette of the Japanese Patent Application Laid-open (JP-A) No. 2000-284206 by the present inventor, a plural-focal-point-producing means for dividing a beam of light outputted from a light source into a plurality of beams of light and for producing a plurality of focal points on a recording medium with respect to the sub scanning direction of the recording medium by using a light-condensing optical system, and a sub-scanning-controlling means for controlling the recording interval in the sub scanning direction according to the resolution are provided. As a result, the number of the focal points, which are produced by the plural focal point producing means by division of the beam of light in the sub scanning direction according to a desired resolution of the recorded image at the time of image recording by condensing the light outputted from the light source on the recording medium via the light-condensing optical system, is controlled in order to adjust the size of the beam spot and to adjust the recording interval of the beam spot in the sub scanning direction. This enables efficient recording of an image at the desired resolution.
In order to improve recording speed, there is an exposing-recording device in which laser beams outputted from a plurality of light sources are each guided to a single exposure head by an optical fiber and in which laser-beam outlets, which are each disposed at an end of one of the optical fibers at the exposure head, are provided side by side for simultaneously executing the exposure by the plurality of the laser beams outputted from the plurality of the light sources.
When the above-mentioned technique disclosed in JP-A No. 2000-284206 is used in this kind of the exposing-recording device, exposure with a laser beam divided into a plurality of laser beams is possible, and thus further improvement in recording speed can be achieved.
However, according to the above-mentioned-exposing-recording device using the optical fibers, the polarization direction of the light outputted from the optical fibers may change over time due to displacement by external forces applied to the optical fibers (including vibration, pressure, and distortion), temperature displacement, or the like as shown in FIG. 14. In this case, since the light is divided unevenly among the plurality of beams so as to make the focal spots unstable, there is a problem in that the quality of the recorded image is deteriorated.
In other words, as shown in FIG. 15A as an example, in a case where two focal points are produced by the above-mentioned plural focal point producing means by using as the light source a semiconductor laser, with an intensity distribution having a high central light intensity, and the light intensity is gradually lowered as it moves away from the center, it is ideal in terms of image quality, to have the intensity distribution of the two resulting beams of light correspond to the two focal points in the same state as shown in FIG. 15B.
In a case where the laser beam polarization direction is changed over time as mentioned above, however, since there is a risk of the intensities of the two resulting beams of light corresponding to the focal points becoming drastically different as shown in FIG. 15C, the image quality of the recorded image may be deteriorated. According to an experiment conducted by the present inventor, it was found that the polarization ratio of the horizontal polarization and the vertical polarization (horizontal polarization:vertical polarization) is changed in a range of 1:4 to 4:1 when an optical-fiber-coupled semiconductor laser is used as a light source.